Method of forming a point at the end of a wire



Jan. 13, w PFANN v 2,434,286

A METHOD OF FORMING A POINT AT THE END 0F A WIRE Filed Aug. 12, 1943 2Sheets-Sheet 1 Till/N6 9 IN VE N TOR H. G. PF4NN WWMM A TTORNEV Jan. 13,1948. I w. PFANN METHOD OF FORMING- A POi-NT AT THE END or A WIRE FiledAug. 12, 1943 2 Sheets-Sheet 2 FIG. 2

lNl EN TOR M. G. PFAN/V A TTORNEV Patented Jan. 13, 1948 METHOD OFFORMING A POINT AT THE END OF A WIRE William G. Pfann, Summit, N. J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application August 12, 1943, Serial No.498,376

2 Claims. (Cl. 204-142) This invention relates to electrical conductingand translating devices and particularly to methods of making them.

The'objects of the invention are to obtain a higher degree of uniformityand to realize greater economy and simplicity in the manufacture ofconducting and translating devices; to increase their electricalefiiciency; and to obtain other improvements in these devices and in themethods of making them.

To a very large extent the success with which ultra-high frequencysignals, including those corresponding to wavelengths of a fewcentimeters, are employed in the radio and allied arts depends on thedevelopment of a. translating device which is capable of detecting,converting, translating or otherwise utilizing signal waves of theseextreme frequencies. Up to the present time the most promisingtranslating device for these purposes is one of the point-contact type.In one of its forms it includes a fine tungsten wire mounted with thefree end of the wire engaging the surface of an element having suitablerectifying properties such as a crystal of elemental silicon.

More specifically, one method heretofore used in making these rectifiersconsists in grinding one end of the fine tungsten wire to a sharp point,soldering the other end of the wire into the end of a metal rod orholder, and carefully assembling the elements to bring the free ends ofthe wire first into proximity with but not touching the crystal surface.Thereafter the holder is adjusted until the point of the wire bearsagainst the crystal with just the right degree of force. And it may benoted at this point that a slight excess of force destroys the contactpoint. Another method is to polish the point of the wireelectrolytically in addition to the grinding step. There are objectionsto both of these methods. Both require the grinding operation which mustbe done with care and precision. Also in both cases the specificationdimensions of the holder to which the contact wires are soldered must beclosely met in the manufacturing process, which increases the cost, andany deviation from the specified dimensions necessitates a greaterdegree of care in the assembling operation. Otherwise the tip end of thecontact wire is brought inadvertently into engagement with the crystalwith excessive force, resulting in the destruction of the point.

In accordance with the present invention these disadvantages aresurmounted by means of a new method of forming points on contact wireswhich eliminates entirely the preparatory grinding operation and whichautomatically controls to very close limits the length of the contactwire in its supporting holder. More specifically a section of tungstenWire is cut to a length somewhat greater than that ultimately required,an approximation being sufiicient, one end of this wire is soldered intoa. metallic holder, and the other end is treated in a specially preparedelectrolytic solution to form a contact point of the desired shape andat a distance from the end of the holder, or from any other referencepoint thereon, which is exactly predetermined. This method not onlydispenses with the preliminary grinding of the wire, a. time-saving stepin the process, but it also substantially eliminates the danger ofdestroying the contact during the assembling operation. Moreover, thislatter achievement is realized without narrowing the limits of variationpermissible in the manufacture of the contact holders, thus obviatingthe adoption of costly precision methods in making these holders. Infact the variation limits may be relaxed in some respects.

A feature of the invention is the method of shaping the end of a wire inwhich a portion of the wire is immersed in an etchant solution and inwhich the meniscus formed around the wire at the surface of the solutionis controlled during the etching process to determine the shape of thepoint formed. -This control over the meniscus is obtained by adding aparticular inredient to the etchant solution.

The foregoing and other features of the invention will be described morefully in the follownig detailed specification. I

In the drawings accompanying the. specification:

Fig. 1 illustrates the electrolytic apparatus for etching the contactwires;

Fig. 2 is a, view illustrating the effect of the electrolyte on thecontact wire; and

Fig. 3 illustrates the contact wire at diiierent stages in the etchingprocess.

Fig. 4 is a view partly in section of the fully assembled translator.

In one of these translating or rectifying devices designed forultra-high frequency waves the critical part of the structure is thesmall contact point at the end of the contact wire which makesengagement with the surface of the rectifying crystal. To betterunderstand the minute size of this contact point it may be noted at theoutset that the tungsten contact wire used in these translating orrectifying devices has a diameter which is preferably as small as .005inch and that the end of this wire is ground and shaped to form 3 acontact engagement surface which is still smaller in diameter. Afterthis contact point is formed on the end of the wire, the unit isassembled, and the contact point is advanced with care and precisioninto physical engagement with the surface of the crystal until the forceexerted by the contact wire against the crystal is of just the rightmagnitude' In this manner the rectification contact is attained, and ifexcessive force is inadvertently applied the contact is ruined, and thewire must be discarded.

Before proceeding with a description of the process which constitutesthe invention herein, a brief explanation will be given of the practicenow followed in the preparation and assembling of these translatingdevices. Referring to Figs. 1 and 4 the tunsten wire is first cut to theestimated required length, following which this piece of wire I issoldered into the bore in an end of the cylindrical metal holder 3.Thereupon the tungsten wire I is given the shape shown in Figs. 1 and 4to give it resiliency. There are a number of factors which tend to varythe distance d between the end face of the metal holder 3 Fig. 4 and thetip end of the contact wire I. In the first place the bore holes in theends of the holders will be subject to the usual manufacturingvariations. Secondly, the amount of the wire I extending into the boreduring the soldering operation will vary from one unit to another.Finally, the amount of wire used in the spring formation will also varyfrom unit to unit. Since it is desirable in the initial assembly to havethe tip end of the contact wire I spaced only a short distance from thesurface of the rectifying crystal, preparatory to the final adjustment,this variation in the distance d results in a large percentage of thecontact wires being destroyed in the initial assembly. The method used,which is the most convenient one available, for performing the initialassembling operation is to insert the cylindrical holder 3 into the bore4 of the cap member 5 and to adjust the set screw 8 until the end 6 ofthe holder is flush with the surface of the integral stud I. Thereuponthe set screws 9 are tightened enough to prevent any displacement of theholder 3. Next the threaded stud I is screwed into one end of the hollowceramic cylinder I0, following which the base member II is added to theassembly. The base member I I includes an integral metallic stud I2which serves as the support for the silicon wafer I3. When, therefore,the threaded stud I2 is screwed into the opposite end of the ceramiccylinder II] the enclosure within the cylinder is hidden from theoperators view, and the position occupied by the tip end of the contactwire I with respect to the surface of the silicon wafer I3 depends uponthe distance d. If this distance is too great, the wafer I3 will engagethe contact point and destroy it when the stud I2 is screwed into place.

Applicants method of forming points on contact wires, whereby theforegoing difficulties are obviated and the processes heretofore usedfor making translator contacts are simplified, will now be described indetail. The initial steps are the same; the contact wires I are cut andsoldered into the bore holes in the ends of the metal holders 3, and thewires are formed for resilience. The grinding operation, however, isomitted. A plurality of these units are now secured in a metallic holderI4 Fig, 1 which comprises two metal strips hinged at one end andcontaining a series of semicircular grooves, each pair of 4 whichreceives one of the contact wire units. In assembling the units in theholder I4, a uniform distance between the lower edge I6 of the holderand the ends of the units 3 may be achieved by using a solid metalholder having holes drilled therein to a uniform depth for receiving theunits 3. The holder I4 is now placed on a dish I9, containing anelectrolyte, with the contact wires projecting downwardly. The dish I9may be supported on a tripod having leveling screws, for adjusting thelevel of the electrolyte solution within the dish with respect to therim on which the holder I4 is supported.

The etchant electrolyte in the dish I9 is preferably an aqueous solutionof potassium hydroxide containing an ingredient, to be explainedhereinafter, for controlling the meniscus formation at the surface ofthe solution. The metal holder I4 is connected through an ammeter 24 andpotentiometer 25 to the positive pole of the source 26 causing thecontact wires to serve as anodes in the electrolytic action. The cathode21 is preferably formed of copper gauze and is connected throughsuitable switches 34 and 35 to the negative pole of the source 26.Before using, the copper gauze is permitted to remain in the solutionlong enough to acquire an oxide coating.

The level of the electrolyte within the dish I9 is controlled by anysuitable means such as a pump and siphon arrangement, and it may beassumed that the level within the dish is relatively low when the holderI4, filled with contact wire elements, is placed in position on the rimof the dish. The level of the electrolyte may then be brought to thedesired elevation for immersing the ends of the contact wires.

The first step in the point-forming process is to etch away the immersedsection of each contact wire until a point of the desired configurationis formed thereon. As the electrolytic action proceeds, the majorportion of the wire below the surface of the electrolyte retains ingeneral its cylindrical form and gradually diminishes in diameter, asillustrated in Figs. 3a, 3b, 3c and 3d. However, the short section ofthe wire which lies within the zone of the meniscus 36 Fig. 2 is etchedat a much lower rate, which decreases as the surface of the electrolyteis approached, becomin nil at the surface. The result of this phenomenonis the formation of a tapered section inside the meniscus as seen inFig. 2. Finally the diminishing cylindrical section is wholly dissolvedor is severed just before complete solution, leaving a sharply pointedtaper as illustrated in Fig. 3e. Further action of the electrolytecauses the tapered point of Fig. 3e to become more and more roundeduntil finally, if the action is permitted to continue, the end of thewire assumes a substantialy plane surface. Thus it is possible to selectbetween the limits of Fig.

3e and a plane-ended wire any desired one of a large number of shapes.

Since the shaping of the point depends on the shape of the meniscus 36,it is important to control the extent of the meniscus and to insure itsmaintenance during th process. For wires of the s'ze above mentioned(.005 inch in diameter) it has been found that a solution of potassiumhydroxide, 25 per cent by weight, provides a meniscus of suitable form.But a meniscus formed by this liquid is too unstable for the purpose; itis adversely affected by transit vibrations and other disturbances.Applicant has discovered, however, that this obstacle may be overcome byintroducing a certain amount of copper into the electrolyte solution.When this is done the sensitivity of the meniscus to outsidedisturbances is greatly reduced, it remains stable throughout theelectrolytic process and controls the formation of the points as aboveexplained. The introduction of copper into the electrolyte may beaccomplished by using a cathode 2'! of fresh copper gauze. A moresatisfactory meth- 0d, however, is to prepare a stock solution which maybe added in proper amounts to the electrolyte. Such a solution may bemade by dissolving 2 grams of cupric chloride (CLICI2'2H2O) in 375 cubiccentimeters of distilled water and then adding 125 grams of potassiumhydroxide. some of the copper remains in solution while the reminderprecipitates as copper hydroxide, which in turn immediately changes to ablack precipitate of cupric oxide. The precipitate is allowed to settle,and the blue solution, after being decanted, is used for the stocksolution to control the electrolyte meniscus. About 30 cubic centimetersof the stock solution added to 600 cubic centimeters of potassiumhydroxide electrolyte is found to give good results, although otheramounts may be used with success.

Although a wide variety of contact point shapes may be obtained withthis process, satisfactory rectifier performance may be had when atungsten wire of .005 inch in diameter is etched until the curvature atthe tip of the point is about .0003 inch in radius. It is possible toattain these and other desired dimensions with considerable accuracy byreason of the fact that the current flowing during the electrolyticprocess decreases continuously. As the surface of the immersed wiredecreases in area, the resistance to the flow of current increases. Alsothe effect of polarization is to decrease the amount of current flowing.While these current values will vary for different electrolytes and forwires of different sizes, it has been found that a point of the desiredshape may be obtained under the conditions herein assumed by using avoltage of 1 volt and continuing the electrolytic action until thecurrent drops to a value between 0.25 and 0.30 milliampere per wire.

Referring again to Fig. 1, this result may be obtained by adjusting thepotentiometer 25 until the meter 32 indicates 1 volt, closing the switch35 and observing the ammeter '24 until it indicates the desired currentper wire at which time the process is stopped. Thereupon the liquidwithin the dish 19 is lowered for a time, following which it is againelevated until the contact points are immersed. Finally the formedpoints are given several flashes at a higher voltage, say 1.9 volts, fora measured interval, say 0.3 second. This is accomplished by a timingswitch 34 which will hold the circuit closed for the desired intervaland then will open it automatically. Th purpose of this flashing step isto give the formed point a high polish which greatly increases theeffectiveness of th wire when used in the rectifier.

The advantage of this method in the manufacture of rectifier assemblieswill now be apparent. The pointing operation in the electrolytic bathinsures that all contact wire units hav exactly the same distance d fromthe end of the holder 3 to the tip end of the point. Therefore, when theholders 3 are subsequently inserted in the cap 5 and these caps arescrewed into the ceramic cylinders ID the uniformity of the distance dinsures the proper location of the contact point with respect to thecrystal surface i3 when the cap H is screwed into place. Anotheradvantage of this process is that it insures uniformity in the shapingand polishing of the contact points, and, as hereinbefore mentioned, iteliminates the grinding operation heretofor necessary in the formationof these points.

Although tungsten wires give excellent results when pointed by themethod herein described, it will be understood that this method may alsobe used to point wires of other materials, such as molybdenum.

What is claimed is:

l. The method of shaping an electrical contact wire which comprisesimmersing one end of the wire below the free surface of an aqueousolution of potassium hydroxide to form a meniscus at the surface ofsaid solution, applying an electrical potential to said wire to causethe etching thereof by electrolytic action and the formation of asymmetrical point in the region of said meniscus, and adding a solublecopper compound to said solution for the purpose of sustaining aidmeniscus while said symmetrical point is being formed.

2. The method of forming a point at the end of a tungsten contact wirewhich comprises immersing a section at one end of the wire in a 25 percent solution of potassium hydroxide in water to form a meniscus aroundsaid wire at the point of entrance at the surface of the solution,anodically treating the immersed wire below the region of aid meniscusto form within the region of said meniscus a symmetrical point on saidwire, and having copper dissolved in said solution to stabilize saidmeniscus during the formation of said point.

WILLIAM G. PFANN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

